Hemicelluloses are heterogeneous polymers of pentoses (e. g., xylose and arabinose), hexoses (e. g., mannose, glucose, and galactose), and sugar acids. Unlike cellulose, hemicelluloses are not chemically homogeneous. Hemicelluloses are relatively easily hydrolyzed by acids to their monomer components consisting of glucose, mannose, galactose, xylose, arabinose, and small amounts of rhamnose, glucuronic acid, methylglucuronic acid, and galacturonic acid. Hardwood hemicelluloses contain mostly xylans, whereas softwood hemicelluloses contain mostly glucomannans. Xylans are the most abundant hemicelluloses. Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of 1,4- linked ^-D-xylopyranose units. Xylans from different sources, such as grasses, cereals, softwood, and hardwood, differ in composition. Besides xylose, xylans may contain arabinose, glucuronic acid or its 4-O-methyl ether, and acetic, ferulic, and p-coumaric acids. The degree of polymer­ization of hardwood xylans (150-200) is higher than that of softwoods (70-130) [14, 15].

1.5.2 Lignin

Lignin is a very complex molecule. It is an aromatic polymer constructed of phenylpropane units linked in a three-dimensional structure. Generally, softwoods contain more lignin than hardwoods. Lignins are divided into two classes, namely, “guaiacyl lignins” and “guaiacyl — syringyl lignins.” Although the principal structural elements in lignin have been largely clarified, many aspects of their chemistry remain unclear. Chemical bonds have been reported between lignin and hemi — cellulose, and even cellulose. Lignins are extremely resistant to chem­ical and enzymatic degradation. Biological degradation can be achieved mainly by fungi, but also by certain actinomycetes [15, 17].